This invention relates to ionically cross-linked siloxane polymers. More particularly, this invention relates to a method of dissociating ionically cross-linked zwitterionic siloxane polymers and also reforming the ionic cross links between dissociated zwitterionic siloxane polymers, herein referred to as "recuring".
Zwitterions are ions which are both positively and negatively charged. Common zwitterionic species are the amino sulfonates, NH.sub.2.sup.+ --R--SO.sub.3.sup.- and the amino carbonates, NH.sub.2.sup.+ --R--COO.sup.- ; wherein R is a divalent hydrocarbon radical more particularly defined below. Zwitterionic species are typically obtained from ionizing amino acids and the like; however, siloxane polymers containing zwitterions have been prepared by Litt and Matsuda, J. Polymer Science, Vol. 19, p. 1221 (1975) and by Graiver et al., J. Poly. Sci., Polymer Chem. Ed., Vol. 17, p. 3559 (1975). The contents of these articles are incorporated by reference herein.
Litt and Matsuda disclose a process for producing zwitterionic silanes by reacting the trifunctional aminoalkyl silanes, .gamma.-aminopropyltriethoxysilane and N-aminoethyl-8-amino-propyltrimethoxy silane, with .gamma.-propane sultone.
Graiver et al. disclose that siloxane polymers containing zwitterions can be obtained by treating an aminoalkyl siloxane with .gamma.-propane sultone. The aminoalkyl siloxanes are provided by copolymerizing a dimethoxy silane having an aminoalkyl radical with a low molecular weight hydroxy-terminated polydimethylsiloxane and decamethyltetrasiloxane.
The zwitterions on the siloxane polymers provide ionic cross-linking between the siloxane polymers due to the coulombic forces exerted by the ions. An example of an ionic cross link which may exist between two siloxane polymer segments is illustrated in the following formula: ##STR1## wherein R' is a divalent hydrocarbon radical of from 1 to 20 carbon atoms and R is a divalent hydrocarbon radical of from 2 to 20 carbon atoms.
These cross-links reduce the mobility of the polymer segments and increases their stiffness. For example, polydimethylsiloxanes (DP=500) are typically liquid at room temperature, yet corresponding zwitterionic polysiloxanes are solid rubbers at this temperature. Introducing zwitterions to as few as 0.5% of the silicone atoms within a siloxane fluid will provide a solid elastomeric material.
These elastomeric materials exhibit high adhesion to glass and other substrates such as, for example, wood, metal, polycarbonates, polystyrene, polyphenylene oxides and blends thereof, etc. The elastomeric properties and adhesive properties of these zwitterionic siloxanes make them suitable for use as adhesives, elastomeric adhesives, sealants, coatings, injection moldable and compression moldable rubbers and plastics, and various silicone based rubbers.
Once the ionic cross-links are formed, it is very difficult to solubilize the siloxanes in an organic solvent. For example, toluene is a suitable solvent for most polydimethylsiloxanes. However, toluene will not solubilize the zwitterionic siloxanes even when utilized in large volumes at its reflux temperature over extended periods of time.
The solubility of the zwitterionic siloxanes is an important property in determining the applications of these materials. For example, the ability to remove a coating once applied is an important feature which permits replacement of the coating when desired. The ability to solubilize the zwitterionic siloxanes is also important when utilizing these materials as adhesives. Solubilizing the adhesives permits the separation of the adhered parts, permitting replacement or reorientation of the adhered parts and adhesive.
The present invention is based on the discovery that the ionic cross-links of zwitterionic polysiloxanes can be dissociated without degradation in the presence of a weak base, permitting the zwitterionic siloxanes to be solubilized in an organic solvent, and that these ionic cross-links can be reformed upon the removal of the weak base from the zwitterionic siloxanes.